Fall Fertilizer Application: Best Practices to Save Time, Money and Nature

Iowa farmer uses regenerative practices to build soil organic matter, decreasing need for fall fertilizer.

Kelly Nieuwenhuis rarely applies fertilizer in the fall on the 2,700 acres of corn and beans he farms with his brothers in O’Brien County, Iowa. The third-generation farmer applies fertilizer in the spring, and his farming practices enable him to apply far less than the recommended amount.

“We leave 90 percent of our acres untouched in the fall,” he said, “and over the winter the corn stalks and everything deteriorate so much that we can hit it once with a vertical tillage tool [in the spring] and then plant soybeans right into that.”

Nieuwenhuis began one-pass vertical tillage seven years ago, but his family started reducing tillage back in the 1990s.  As a result, his soils are rich in organic matter. In some fields, organic matter content rises as high as 6.5%.

Confidence in his Soils’ Fertility Led Nieuwenhuis to Stop Applying Fall Fertilizer

“We were seeing our organic matter being built, and we knew there was extra nutrient holding capabilities and better water holding capabilities, and we saw the credits available there for our next growing season,” he said.

Cutting out the fall fertilizer application not only saves Nieuwenhuis considerable time and money, it’s also good for the environment and water quality. Nutrient loss from farms can be especially problematic in the fall if fertilizer is applied on frozen or saturated ground. Snow and rain wash away nutrients that are unable to percolate through the soil.

Building soil organic matter is one way to minimize nutrient loss in the fall and winter months. Experts say there are other more immediate steps that farmers can take. These include:

  • Assessing soil nitrate levels with a soil test before deciding to make fall applications. A soil test can help reduce overall application amount or potentially show that an application is unnecessary.
  • Using a stabilizer or a nitrification inhibitor to keep nitrogen longer in a usable form and ensure soil temps are below 50 F degrees before making any fall applications
  • Taking advantage of residual soil nitrate and considering split applications, including in season applications at the v3 or v6 stage for instance.
  • Planting a cover crop
Eliminating fall fertilizer application can save farmers money and benefit the environment. Nutrient loss from farms can be especially problematic if fertilizer is applied on frozen or saturated ground. © Chris Helzer/TNC

“Ideally, we are not applying nitrogen in the fall, but if a grower is considering a fall application, it is important to verify existing soil nitrate levels with a soil test to account for any residual nitrogen that is left in the soil,” said Paige Frautschy, director of agriculture at TNC Iowa.

Recommendations for Keeping Nitrogen in the Soil

Frautschy notes that it’s particularly important to test soil in areas where there has been significant drought because soil retains more nitrogen during a drought year. Crops use less nitrogen, but there is also a major reduction in the amount of nitrogen lost to leaching and denitrification, according to Michael Castellano, professor of agronomy at Iowa State University.

The second recommendation is to make sure that all fall nitrogen applications have a nitrification inhibitor, or a stabilizer, and are applied when the soil temperature is 50 F degrees or less, to keep the nitrogen in a usable form longer and prevent losses from volatilization, denitrification or leaching.

Nieuwenhuis has been using a stabilizer with his urea applications for the last five years and says he’s very happy with it. “We’ve been pretty consistently raising 240 to 260 bushels of corn on 140 pounds of nitrogen, spring applied, [with it]” he said.

Two different types of nitrogen stabilizer products are available: urease inhibitors and nitrification inhibitors.  Urease inhibitors, like the one Nieuwenhuis uses, prevent urea from converting into ammonium, which can lead to losses through the ammonia gas that’s produced. Such losses can occur within hours of applying urea-containing fertilizer. Once urea is incorporated into the soil, through tillage, injection, or light rainfall, ammonia volatilization is greatly reduced.

Nitrification inhibitors are chemical compounds that temporarily reduce populations of the bacteria in soil (Nitrosomonas and Nitrobacter) that convert ammonium to nitrate, which can easily leach out of soil. These compounds also prevent denitrification, or the microbial process that turns nitrate to nitrogen gas, causing further nutrient loss.

The third recommendation is to take advantage of residual soil nitrate and utilize split fertilizer application during the growing season.

Castellano says that a little more than half a pound of nitrogen is removed in each bushel of corn harvested. That means, for example, “for a farmer that applies 200 pounds of nitrogen an acre and has 200 bushels per acre corn yield, there could be nearly 100 pounds of nitrogen still residing in the soil.”

Factoring in that residual nitrogen into fall fertilizer planning is “important for water quality since it reduces potential nutrient losses – especially if we have heavy fall or spring rains following a drought year. We’re also seeing record high fertilizer prices, so its beneficial for the farmer’s bottom line too,” said Frautschy.

Reservoir of Nutrients

Research also shows that each percent of organic matter in the soil releases 20 to 30 pounds of nitrogen, 4.5 to 6.6 pounds of P2O5, and 2 to 3 pounds of sulfur per year.  Growers who have built the organic matter in their soils can take advantage of that reservoir of nutrients without depleting the reserves.

Splitting the spring fertilizer application by applying a starter fertilizer and the rest when the plant is actively growing can help prevent nutrient loss from heavy rainfall events.

“There’s a better chance that the crop will actually be able to use that fertilizer in the spring, during the growing season, versus if you’re applying it in the fall,” noted Frautschy, adding that fertilizer might be cheaper or easier to apply in the fall, but it is also vulnerable to loss as it is exposed to freeze thaw cycles, rain, etc. These losses may cancel out any potential cost savings.

Last, experts recommend planting a fall cover crop, like winter rye. “Cover crops can easily retain 25 pounds of nitrogen per acre or more by scavenging and holding excess nitrogen in the soil and releasing it back as they decompose in the spring, making it available for future crops to take up. Cover crops are also great tools for building soil health and for protecting our valuable soils from wind and water erosion.

Nieuwenhuis is running a trial of cover crops, comparing no till and cover crops to minimal till with cover crops, and evaluating the impact on weed pressure, herbicide use, and nutrient holding capacity, as well as on yield.

“I truly believe our soils are our best filter system we can have and if you got a good organic matter and your water is going through that, that organic matter holds the nutrients,” he said.

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